1. Field of the Invention
The present invention relates to an improved magneto-optic head for reading and recording a magnetooptic disc, which constitutes an external semiconductor laser resonator in which the magneto-optic disc is used as a reflection plane.
2. Description of the Related Art
Recent remarkable developments of an optical disc system, particularly employing an optical disc as a recording medium thereof, and in which data is recorded, reproduced and erased using a laser beam, have brought about a wide spread use of compact discs (CD) and a CD-ROM as a Read Only Memory for personal computers. Furthermore, an application of the optical disc system to office automation (OA) equipment or other information-related equipment, as a mass storage unit, is expected. In particular, a rewritable optical disc system is being developed to provide a memory capacity larger than the capacity offered by the magnetic disc system most commonly used as a file memory.
To enable the use of a magneto-optic disc as a rewritable optical disc system, there is the need for a development of an improved magneto-optic recording and reading head having a high S/N and ensuring stable reading.
Data is recorded on a surface of a magneto-optic disc by changing the direction of magnetization at small spots (pits) so that, when a light is made incident thereon, the polarization plane of the reflected light is subject to the Kerr effect, i.e., rotation to the right or left depending on the direction of the magnetization.
The reflected light is detected by a detector, after it is transmitted through a polarizer (analyzer), to read the recorded data, but usually, since the angle of Kerr rotation is very small, the quantity of signal light which can be utilized thereby is very small, e.g., a maximum of 0.1% of the total quantity of light incident upon the magneto-optic disc, and thus the signal light is easily influenced by noise.
To solve this problem, it is known that the reflected light can be split into two beams by a polarization beam splitter, the two split light beams being received by two respective detectors and detected by a differential detection method to thereby read the recorded data with less noise and a high S/N. Nevertheless, a differential detection method using two detectors complicates the optical system and the electrical signal processing system.
Further, attempts have been made to read the recorded data by using an external semiconductor laser resonator in which the magneto-optic disc constitutes one of the reflection planes thereof. For example, Japanese Unexamined Patent Publication (Kokai) No. 1-182947 (corresponding to USSN 295,753) discloses such an external semiconductor laser resonator in which the recorded data is read by detecting, for example, beat signals, as shown in FIG. 12.
In FIG. 12, 2 designates a magneto-optic disc having a magnetic recording layer in which data is recorded by magnetization in one direction (shown by arrows), and 1 denotes a semiconductor laser provided, on the opposite end surfaces of which are formed anti-reflection coatings 10 and 11. A first quarter wave plate (.lambda./4 plate) 5 is provided between the magneto-optic disc 2 and the semiconductor laser 1.
The magneto-optic recording and reading head also includes a second quarter wave plate (.lambda./4 plate) 6, a Faraday rotation element 7, an output mirror 4 through which light can be partly transmitted, a polarizer 8, and an optical detector 9 such as a PIN photodiode, on the opposite side of the semiconductor laser 1 to that of the first quarter wave plate 5, in this order. The magneto-optic recording and reading head further includes a first collimating lens 14 provided between the laser 1 and the first quarter wave plate 5, an objective lens 13 provided between the quarter wave plate 5 and the magneto-optic disc 2, and a second collimating lens 15 provided between the laser 1 and the second quarter wave plate 6.
In this optical system, light emitted from the laser 1 oscillates through the .lambda./4 plate 5.fwdarw.the magneto-optic disc 2.fwdarw.the .lambda./4 plate 5.fwdarw.the laser 1.fwdarw.the .lambda./4 plate 6.fwdarw.the Faraday rotation element 7.fwdarw.the output mirror 4.fwdarw.the Faraday rotation element 7.fwdarw.the .lambda./4 plate 6.fwdarw.the laser 1, to perform a laser oscillation.
During this laser oscillation, for example, a linear polarization light 1 in a TM mode (in which an electric field component exists in a plane perpendicular to an active layer of the semiconductor laser) transforms the mode thereof in the following manner, 2 .fwdarw.3.fwdarw./e,crc/4/ , and is returned as a linear polarization light 4 in a TE mode (in which an electric field component exists in a plane parallel to an active layer of the semiconductor laser). The returned light (TE mode) is then incident upon the output mirror side and transforms the mode thereof like in the sequence 5.fwdarw.6.fwdarw.7.fwdarw.8 and is returned as a TM mode 8. A similar laser oscillation occurs for the modes (not shown) perpendicular to the modes 1-8.
Namely, two polarization modes perpendicular to each other exist, and are simultaneously generated in the resonator. In theory, the two modes are absolutely the same as each other and have the same wavelength (oscillation frequency), but due to the difference in phase between a right (or clockwise)-rotated circular polarization light and a left (or counterclockwise)-rotated circular polarization light, caused by the Kerr rotation of the magneto-optic effect of the two mode signals on the magneto-optic disc, a variation of the resonator length (cavity length of the resonator) occurs in practice, thereby resulting in a slight change of the oscillation frequency of the two mode signals.
When the two laser beams having different frequencies are superimposed, a beat signal corresponding to the difference between the frequencies is produced, and this beat signal can be detected by the optical detector 9 to thereby read the recorded data.
Assuming that the Kerr rotation angle is .theta..sub.1 (radian) when a beam spot is incident upon the recording pit having a predetermined magnetization direction, a beat frequence f.sub.1, is expressed by the following formula (see JPP' 947 mentioned above): EQU f.sub.1 '=c.theta..sub.1 /2.pi.L . . . (1)
wherein, c is the speed of light (c=3.times.10.sup.9 m/sec), and L is the effective cavity length of the resonator (i.e., the distance between the magneto-optic disc and the output mirror).
When the magneto-optic disc 2 is rotated, so that a beam spot is incident upon a recording pit in a magnetized area having an opposite magnetization direction, the rotation angle of the Kerr effect becomes .theta..sub.2, and thus the beat frequency f.sub.1 ' is changed to f.sub.2 ', as expressed by the formula (1). The difference of the beat frequencies f.sub.1, and f.sub.2 ' makes it possible to read the recorded data.
Since the Faraday rotation element 7 is inserted in the light path, even if a plane mirror is used in place of the magneto-optic disc 2, the beams 2 and 2' (not shown) having a circular polarization light and perpendicular to each other, are subject to a bias rotation of angle 2.theta., and thus a beat signal having, for example, a beat frequency F obtained by the formula (1), is produced.
On the other hand, when the magneto-optic disc 2 (i.e., rather than the plane mirror) is located at the reflection plane, as shown in FIG. 12, the light is further subject to the Kerr rotation of an angle +.DELTA..theta. or -.DELTA..theta., depending on the direction of magnetization thereof, and thus the resultant angle of the bias rotation becomes 2.theta..+-..DELTA..theta.. Therefore, the beat frequency of the beat signal from both is: EQU F.+-..DELTA.F . . . (2)
Consequently, the circular polarization lights 7 and 7' (not shown), perpendicular to each other and emitted from the output mirror 44, are converted to two beams of light having the same plane of polarization, through the polarizer 8, and are then made incident upon the optical detector 9 as a light beat signal to thereby permit detection of the recorded data.
In the prior art mentioned above, however, since the TE mode light and the TM mode light pass in the laser 1 in the same direction, the two modes of light tend to be united and influenced by one another, so that the light power of one of the mode lights moves to the other mode light.
FIG. 11A and 11B show experimental data of the vertical oscillation modes of the prior art, in which the ordinate designates the light output and the abscissa the oscillation frequency. In the measurements shown in FIGS. 11A and 11B, a plane mirror was used instead of the magneto-optic disc 2, in the optical system shown in FIG. 12, to produce a laser oscillation. The laser beam emitted from the output mirror 4 was split into the TE mode light (represented by Mode I) and the TM mode light (represented by Mode II) by a polarization beam splitter and made incident upon a Fabry-Perot interferometer, so that the interference spectrum of light emitted from the Fabry-Perot interferometer was measured and recorded by a light power meter. Note that "FSR" in FIG. 11A stands for Free Spectral Range of the Fabry-Perot interferometer and was 20 GHz.
In FIG. 11A, the plane mirror, used instead of the magneto-optic disc 2, was located in parallel with the output mirror 4, and thus the output of Mode II is much smaller than that of Mode I.
On the other hand, in FIG. 11B in which the plane mirror was fixed and the output mirror 4 was inclined thereto at a slight inclination angle to vary the oscillation conditions, the output of Mode I is much smaller than that of Mode II. These experiments were carried out for different oscillation conditions, and proved that an outstanding imbalance exists between the outputs of Mode I and Mode II, thus resulting in an unbalanced oscillation of the two modes. Namely, it was experimentally confirmed that the light power moves from one to the other of the modes of light.
Where there is a considerable imbalance of the oscillation outputs of the two modes, the intensity of the beat signal between the two mode laser beams becomes low, resulting in a poor S/N.
The primary object of the present invention is to solve the problem mentioned above.